Condensation heat exchanger with plastic casing

ABSTRACT

The invention concerns a heat exchanger comprising at least one helically bound bundle of tubes, whereof the thermally conductive wall has a flattened and oval cross-section, whereof the main axis is perpendicular to that (X-X′) of the helix, while the width of the interstice separating two adjacent turns is small and constant, said bundle being fixedly mounted inside a casing ( 1 ) fitted with a pipe for evacuating gases generated by a burner ( 6 ), means being provided for circulating cold water inside said bundle, said exchanger being further designed so that the hot gases pass radially through the bundle via the interstices of the turns. Said exchanger is characterized in that the casing is made of heat-resistant plastic material and means are provided for ensuring mechanical containment of the beam in the axial direction, and for damping the thrust loads resulting from the fluid internal pressure, preventing their being transmitted to the casing. The inventive condensation heat exchanger is particularly designed for domestic use and is inexpensive to produce.

The present invention relates to a condensation heat exchangerassociated—directly or indirectly—with a burner, particularly a gas orfuel burner.

This exchanger is intended in particular to equip a gas boiler fordomestic applications, with the aim of supplying a central heatingcircuit and/or providing water for sanitary use.

More specifically, the heat exchanger which forms the subject of theinvention is of the type comprising a casing which delimits an enclosureinside which is housed at least one bundle made up of a tube or tubes offlattened cross section, of the kind described in document EP-B-0 678186, to which reference may be made as required.

Document EP-B-0 678 186 describes a heat exchanger element whichconsists of a tube made of highly thermally conductive material, inwhich a heat-transfer fluid, for example water to be heated up, isintended to circulate. This tube is helically wound and has a flattenedoval cross section whose major axis is substantially perpendicular tothe axis of the helix, and each turn of the tube has flat faces whichare separated from the faces of the adjacent turn by a gap of constantwidth, this width being substantially smaller than the thickness of saidcross section, the spacing between two neighboring turns beingadditionally calibrated by means of spacers, these consisting of bossesformed in the wall of the tube.

This document also describes heat exchangers containing a number ofelements, such as described above, which are arranged in different waysin the various embodiments set forth.

An exchanger element thus designed is capable of providing veryefficient heat exchange between, on the one hand, very hot gases whichmay be generated directly by a burner mounted in the enclosure, or comefrom an external source, and which sweep over the tubular element, and,on the other hand, the fluid to be heated up, such as water, whichcirculates inside this tubular element.

Specifically, during its passage through the gap between the turns, inan approximately radial direction, the flow of hot gases comes intocontact with a relatively large area of the wall of the exchangerelement.

The object of the present invention is more particularly to provide acondensation heat exchanger of the general type set forth above, theheat exchange elements of which are bundles of flat tubes such as thosedisclosed in EP-B-0 678 186 mentioned above.

The casing making up the known condensation appliances of the kind setforth above is, just like the tube or tubes, made of metal, generallystainless steel.

Metal, in particular stainless steel, is suitable for use because itprovides both mechanical resistance to the stresses due to expansionsoccurring within the winding made up of a tube or tubes and chemicallyto the corrosion emanating from the flue gases (burnt gases) and thecondensates.

For illustration purposes, it should be pointed out in this regard thatthe pressure of the fluid to be heated up, particularly water, insidethe tube (or tubes) during use may be relatively high, around 2.5 to 3.5bar, that is 2.5·10⁵ to 3.5·10⁵ Pa.

For safety reasons, the tubular bundle is advantageously designed to beable to withstand a pressure of 4.5·10⁵ Pa.

The initially flat lateral walls of the tubes have a tendency to bulge,the amplitude of the deformation being an increasing function of thevalue of the internal pressure.

This deformation is propagated axially, from one wall to the adjacentwall, by way of spacer-forming bosses which separate them.

For illustrative purposes, taking a winding of four juxtaposed tubeshaving a wall thickness of 0.6 mm, the axial dimension of which isinitially 128 mm, this dimension, subsequent to the deformation of thetubes, will be increased to a value of around 129.2 mm for a pressure of2 bar and of around 129.8 mm for a pressure of 3 bar.

The total elongation is proportional to the number of windings mountedend to end constituting the bundle of the exchanger.

Of course, increasing the wall thickness of the tubes can reduce theamplitude of the deformation. Unfortunately, oversizing the thicknessconsiderably increases the weight of the appliance. Problems also ariseif the tubular elements are produced by hydroforming, a processrequiring extremely high operating pressures.

To oppose the elongation and withstand the axial thrusts resulting fromthe internal pressure of the fluid circulating in the bundle, thesolution used up until now is to adopt a metal casing (acting as asupport at the two ends of the bundle), the thickness and mechanicalstrength of which are chosen so that they prevent the axial expansion ofsaid bundle under the effect of the internal pressure, or at leastrestrict it to an acceptable amplitude compatible with the elasticdeformation limit of the casing.

This type of exchanger is satisfactory on the technical level, inparticular on the performance level.

However, it is relatively heavy, which may create difficulties for theoperator when it is being transported and handled during itsinstallation, and its cost price is relatively high because, in order towithstand the mechanical stresses and chemical attacks caused by theflue gases and the condensates, it is necessary to make use of a casingmade of high quality metal, such as stainless steel.

The objective on which the present invention is based is tosignificantly reduce the weight and the cost price of the appliance, byproposing that it be equipped with a casing which, although made of asubstantially less noble and less costly material, in this instanceplastic, does not present any problems either in terms of chemicalresistance or mechanical strength with regard to the axial expansionproblem referred to above.

Another objective of the present invention is, in a variant, to ensurethat the plastic casing is optimally insulated from the heat generatedby the burnt gases passing through the turns of the winding and,correspondingly, to substantially lower the level of the temperatures towhich the casing is exposed, this being achieved by employing simple,lightweight and inexpensive means, in this instance a shroud performingthe function of a heat shield.

The condensation heat exchanger which forms the subject of the inventionis intended to be associated with a gas or fuel burner.

It comprises at least one bundle of tubes, which bundle consists of onetube, or a group of tubes arranged end to end, forming a helicalwinding, in which the wall of the tube or tubes is made of a highlythermally conductive material and has a flattened oval cross sectionwhose major axis is perpendicular, or approximately perpendicular, tothat of the helix, while the width of the gap separating two adjacentturns is constant and appreciably smaller than the thickness of saidcross section, this bundle being mounted fixedly inside agas-impermeable casing, means being provided for circulating a fluid tobe heated up, in particular cold water, inside the tube or tubesconstituting said bundle, this casing having a sleeve for the dischargeof the burnt gases, this exchanger thus being arranged such that the hotgases generated by the burner pass radially, or approximately radially,through said bundle via the gaps separating its turns.

According to the invention:

-   -   on the one hand, said casing is made of heat-resistant plastic,        and    -   on the other hand, the exchanger contains means for mechanically        retaining said bundle in its axial direction, these means being        able to absorb the thrust loads resulting from the internal        pressure of the fluid which circulates therein and which tends        to deform the walls thereof, while preventing these loads from        being transmitted to the casing.

There is thus a dissociation of the two tasks assigned to the casing upuntil now, namely acting as an enclosure for the circulation anddischarge of the hot gases, and also for the collection and discharge ofthe condensates, and, on the other hand, ensuring the mechanicalstability of the bundle of tubes.

Furthermore, according to a certain number of characteristics which areadvantageous but do not limit the invention:

-   -   the exchanger contains a temperature probe borne by said casing        which is able to shut down the burner when the temperature        prevailing inside the casing, in the vicinity of this probe,        exceeds a predetermined threshold;    -   said retaining means comprise a set of ties which extend outside        the bundle, parallel to the axis of the helix, and whose ends        are fixed to bearing elements pressing against the two opposed        faces of the bundle;    -   the bearing element situated at one of the ends of the set of        ties is a thin plate, for example in the form of a disk, which        is cut out in its central part and consequently has an annular        shape;    -   said plate serves as a facing which partially closes off an open        face of the casing and is fastened to the latter at its        periphery, for example by crimping;    -   the end portions of the ties pass through said facing in such a        way as to project slightly outward, and these end portions are        threaded such that they allow a door to be mounted removably        against the facing by means of nuts;    -   said door is fixed to the burner;    -   there are four ties arranged substantially in a square, and the        bearing elements situated on the opposite side to said facing        consist of a pair of arcuate or bent straps configured to follow        the contour of the bundle as closely as possible and pressing        against two diametrically opposed regions thereof, each strap        being fastened to a pair of neighboring ties;    -   the plastic constituting the casing is a composite material        based on glass-fiber-reinforced or glass-flake-reinforced resin;    -   said resin is a compound of polyphenylene oxide, polystyrene and        polypropylene;    -   the exchanger comprises two bundles of coaxial tubes situated        end to end and connected to one another, one of which serves as        a primary exchanger and the other as a secondary exchanger, a        deflecting member being sandwiched between these two bundles and        thus arranged such that the hot gases generated by the burner        pass first through the primary exchanger, passing through the        gaps separating its turns from the inside to the outside, and        then through the secondary exchanger, passing through the gaps        separating its turns from the outside to the inside;    -   the deflector is fixed to said bundles of tubes;    -   since the burner is mounted inside the bundle which serves as        primary exchanger, said deflector has a discoid shape and is        fixed to the end of the burner, this deflector being equipped at        its periphery with a thermally insulating seal which is pressed        against the inside of the bundle;    -   said casing consists of two molded half-shells brought together        and secured to one another, for example by welding;    -   the exchanger contains a shroud arranged outside the bundle made        up of a tube or tubes and inside said plastic casing, this        shroud acting as a heat shield which is able to insulate this        casing from the heat emitted by the burnt gases;    -   this shroud is made from thin stainless steel sheet;    -   the shroud is applied to the internal surface of the plastic        casing but is kept at a certain distance from the latter, for        example by means of a series of bosses stamped into the wall of        the shroud;    -   the shroud consists of two complementary rounded parts brought        together so as to form an annular casing fitting against the        internal surface of said plastic casing;    -   the mutually facing edges of said rounded parts have a row of        approximately semicircular or semioval notches which are able to        tightly enclose the rectilinear end portions of the tube or        tubes constituting the winding when these rounded parts are        brought together.

Other characteristics and advantages of the invention will becomeapparent from the description and the appended drawings which, purely byway of non-limiting example, represent possible embodiments thereof.

In these drawings:

FIG. 1 is a schematic front view of a first embodiment of the invention,cut by the vertical plane referenced I-I in FIG. 2;

FIG. 2 is a schematic left-side view of the appliance of FIG. 1;

FIGS. 3 and 4 are views similar to FIGS. 1 and 2 respectively,representing the bundle of tubes and its retention means only;

FIG. 5 is a view analogous to FIG. 1, representing a second possibleembodiment of the exchanger, the overall axial size of which is smaller;

FIG. 6 is a side view of the exchanger of FIG. 5, illustrating themethod employed therein to retain the bundle;

FIG. 7 schematically represents front views of these retaining means;

FIG. 8 is a detail view showing a possible variant of the temperaturedetector which can be employed, replacing the one illustrated in FIG. 5;

FIG. 9 illustrates the operation of the appliance of FIG. 5;

FIGS. 10, 11 and 12 are views analogous to those of FIGS. 1, 2 and 3,respectively, representing a third embodiment of an exchanger accordingto the invention, not provided with a burner;

FIGS. 13 and 14 are respective schematic front and side views of anexchanger according to the invention, cut by a vertical plane passingthrough the axis of the winding, this exchanger being similar to theembodiment of FIG. 5, but containing a shroud performing a heat shieldfunction;

FIGS. 15 and 16 represent, again schematically, the two strip-formelements (not yet rounded) constituting the shroud.

The exchanger represented in FIGS. 1 and 2 contains a shell or casing 1which delimits an enclosure inside which is fixedly mounted a tubularbundle 2, this consisting of a helical winding, of axis X-X′, of a groupof tubes arranged end to end and connected in series.

These are tubes of flattened cross section whose large sides areperpendicular to the axis X-X′.

Bosses 200 provided on the large faces of the tubes perform the functionof spacers, making it possible to delimit between each turn a gap havinga substantially constant, calibrated value.

This winding is intended to be traversed internally by the fluid to beheated up, which is water for example.

In the embodiment illustrated, there are three helical tubular elementsbrought together and connected in series, in which the fluid to beheated up circulates from left to right.

Manifolds 15, 16, which are fastened to the casing 1, enable theappliance to be connected in the conventional manner to a pipe forfeeding the cold fluid, which is to be heated up, and for dischargingthe hot fluid.

These manifolds also transfer the fluid being circulated from a tubularelement to the neighboring winding.

Each tubular element has straight-end portions, that is to say ofrectilinear axis, with a progressively variable cross section, of whichthe emergent end part is circular.

In the example illustrated in FIG. 2, the two end portions are arrangedparallel and situated on the same side of the winding.

It may be noted that a similar arrangement is also provided for thethird embodiment illustrated in FIGS. 10 and 11.

By contrast, in the case of the second embodiment of the invention,illustrated in FIGS. 5 and 6, the two end portions of a tubular windingextend in the same plane, their mouths being directed away from oneanother, in an arrangement according to that illustrated in FIG. 24 ofEuropean patent 0 678 186 mentioned already.

The inlet and outlet mouths 20, 21 of the tubular elements areappropriately crimped in a sealed manner in ad hoc openings made in thecasing 1, as can be seen from FIG. 2; the manifolds 15, 16 are fastenedat this level.

According to an essential characteristic of the invention, the casing 1is made of plastic.

It is, for example, obtained by rotomolding or injection molding.

The casing is made of two half-shells which are heat-sealed togetherafter the tubular bundle has been installed inside one of them.

The casing 1 is open on one of its sides, in this instance on the sidesituated on the left when considering FIG. 1.

During use of the appliance, a portion of the steam contained in theburnt gases condenses on contact with the walls of the tubes.

The reference 10 denotes the bottom wall of the enclosure; in a knownmanner, this bottom is inclined, thereby enabling the condensates to bedischarged toward an outlet orifice 13.

The rear wall of the casing bears the reference 11; it has a recess 110which, as will be seen later on, forms a channel through which the burntgases and flue gases can pass, channeling them toward a discharge sleeve12.

Of course, the orifice 13 is connected to a condensate discharge pipe,while the sleeve 12 is connected to a flue gas discharge pipe, forexample a flue duct. These pipes and duct are not represented in thefigures.

The open side of the casing is closed off by a facing element 3. Thelatter is fastened over the whole of its periphery by a rim 30 which iscrimped in a gastight manner on a peripheral flange 14 bordering theentrance to the casing.

A seal, for example a silicone seal (not shown), may advantageously beprovided at this level.

The facing plate 3, which is made of stainless steel for example, isnormally closed off by a removable door 4.

In the embodiment represented, the door 4 is in two parts; it iscomposed of an external plate 40, made of heat-resistant plastic ormetal, and of an internal plate 41 made of an insulating, for exampleceramic-based, material.

These two plates are traversed in their central part by an opening whichis traversed by a burner 6, for example a gas burner, which is securedto the door 4 by means which have not been shown.

Suitable means connected to the burner 6 make it possible for a gas andair (such as propane+air) fuel mixture to be fed to the appliance.

These means may consist in particular of a fan fastened to the door andcapable of blowing the gas mixture into the burner, or of a flexiblepipe connected to the door.

The burner 6 is a cylindrical tube with a closed end, the wall of whichis perforated with a multitude of small holes which enable the fuelmixture to pass radially to the outside of the tube.

The outer surface of this wall constitutes the combustion surface. Anignition system (not shown) of known type containing a spark-generatingelectrode, for example, is of course associated with the burner.

The latter is situated coaxially with the center of the winding 2, butit does not extend over the whole length thereof.

In fact, the tubular bundle 2 is subdivided into two parts, one 2 asituated to the left of a deflector 7, and the other 2 b situated to theright thereof.

The deflector 7 is a disk made of a thermally insulating, for exampleceramic-based, material; it is borne by a reinforcement in the form of athin stainless steel plate 70 whose peripheral edge is inserted betweentwo adjacent turns of the bundle.

The exchanger in question is a double exchanger, such as represented inFIG. 8 of the aforementioned European patent, which makes it possible toachieve excellent efficiency.

The part 2 b of the bundle is responsible for preheating the fluid,which circulates from right to left when considering FIG. 1. The part 2a is responsible for the actual heating.

According to an essential characteristic of the invention, the turns ofthe tubular bundle 2 are firmly kept pressed against one another bymeans of a mechanical retaining system.

What is involved in this instance is a set of four ties 5 which areformed by stainless steel cylindrical rods and are associated withbearing elements for each of the two opposed ends of the bundle.

As can be seen from FIG. 2, the ties 5 are arranged at the four verticesof an imaginary isosceles trapezoid. On one side (to the right in FIGS.1 and 3), their end 51 is fastened—for example by welding—to a discoidannular plate 30 made of stainless steel, in the center of which anopening 300 is made.

On the opposite side, which corresponds to the left in FIGS. 1 and 3,the ties 5 are fastened to the facing 3, to which reference has beenmade above.

On this side, the end portions of the ties 5 are threaded; they passthrough suitable orifices made at the periphery of the facing plate 3.

Nuts 500 screwed onto these threaded portions 50 place the ties undertension so as to forcefully apply (from right to left) the plate 30against the last turn of the bundle 2 and, correspondingly (in theopposite direction), the facing 3 against the first turn of this bundle.

The bundle 2 is thus axially compressed with force between the bearingelements 3 and 30.

It will be noted that the end portions 50 are relatively long; theyprotrude beyond the nuts 500 over a considerable length, as can be seenfrom FIG. 3.

The reason for this is that the portions 50 also have the function ofcentering and fastening the door 4 against the facing 3.

To this end, the plate 40 constituting the door, the diameter of whichis greater than the diameter of the insulating part 41, is traversed byfour holes by means of which the portions 50 can be engaged.

The fastening is performed by nuts 400, which are advantageouslyself-locking nuts, to reduce the risk of inadvertent loosening, inparticular under the effect of vibrations.

An annular lip seal 42 housed in a suitable groove made in the plate 40makes it possible to press the latter in a flue gas-tight manner againstthe external face of the facing 3.

As can be seen from FIG. 2, the ties 5 are arranged outside the bundle2.

By observing FIG. 3, it will be made quite clear that the assemblyformed by the facing 3, the ties 5 and the end bearing elements 3, 30forms an independent assembly.

The expansions which tend to occur under the effect of the internalpressure prevailing in the tube of the winding 2 are countered by theties and the bearing elements which fully absorb the axial thrust loads.

There is no transfer of this thrust against the wall of the casingcontaining this assembly.

The tubular bundle can be kept in place inside the casing simply as aresult of the end parts of the tubes 20, 21 being fitted into thehousings provided in the casing to receive them.

It will be noted furthermore, that a deflecting partition 8 is providedabove the rear region of the winding 2, this partition partiallyoverlapping the rear annular plate 30 down to its central opening 300.

This partition advantageously participates in correctly maintaining thebundle inside the casing.

It is fastened to the internal wall of the casing and extends obliquelybelow the sleeve 12. It preferably has an arcuate shape, having acontour forming an arc of a circle, surrounding the upper region of thebundle.

The hot gases generated by the burner 6 pass first through the firstpart 2 a of the bundle 2 (situated to the left of the deflector 7),passing radially between the gaps of the tubes from the inside towardthe outside.

By virtue of the presence of the partition 8, they are unable to escapeimmediately through the sleeve 12.

They must pass through the rear part 2 b of the exchanger (situated tothe right of the deflecting plate 7), this time from the outside towardthe inside, preheating the water which circulates in the tubular bundle.

Finally, the cooled gases escape via the rear channel delimited by thewall 110, to rejoin the discharge sleeve 12.

The plastic constituting the casing is chosen to continuously withstandtemperatures of around 150° to 160° C.

This is advantageously a composite material based on aglass-fiber-reinforced or glass-flake-reinforced resin.

A particularly suitable type of resin which may be mentioned is acompound of polyphenylene oxide, polystyrene and polypropylene, such amaterial being suitable for withstanding chemical attack by hot fluegases and by condensates.

The wall of the casing 1 may be relatively thin, for example between 2and 4 mm thick, owing to the fact that it is not exposed to largemechanical stresses.

For maintenance purposes, access can be easily gained to the inside ofthe front part of the exchanger, this being the only part which isreally exposed to soiling due to the flue gases; all that is requiredfor this is to unscrew the nuts 400 and axially withdraw the assemblyformed by the door 4 and the burner 6 fixed thereto.

After cleaning, it is just as easy to reinstall this assembly.

These disassembly and reassembly operations have no effect on theretaining function performed by the ties 5, which remain active in spiteof the momentary removal of the door.

In a variant embodiment of this device, it would be possible to fastenthe discoid deflector 7 to the end of the burner 6.

In that case, the door 4, the burner 6 and the deflector 7 would form anassembly which could be disassembled en bloc, which would make itpossible to have access for cleaning purposes to the whole of the innerspace of the winding, including the rear portion which performs thepreheating.

Of course, assuming such a situation, it would be necessary to provide ahighly heat-resistant annular seal all around the deflecting disk 7,this seal bearing against the inner surface of the bundle so as toprevent gases passing directly at this level toward the part 2 b.

In the second embodiment of the invention, which is illustrated in FIGS.5 to 7, a configuration analogous to that which has just been describedis once more encountered, although the appliance has been turned aroundby 180° (facing situated to the right of FIG. 5).

The elements which are identical or similar to those of the firstembodiment have been assigned the same reference numbers, and noexplanation with regard to their nature and their function will be givenagain.

It will be noted that this exchanger has greater axial compactness thanin the first embodiment.

As already stated, the rectilinear end portions of the tubes extendtangentially to the winding, their axes being contained in the samelaterally arranged longitudinal plane (see FIG. 6).

Furthermore, on the opposite side to the facing 3, the ties 5 arefastened not to an annular plate 30 but to a pair of bent flat rods 30a, 30 b, the central regions of which bear against an angular sector,having a relatively limited area, of the corresponding end turn.

As can be seen from FIG. 6, the ties are this time arranged in a square,and the bent rods 30 a, 30 b connect these sides in pairs, following asclosely as possible two diametrically opposed regions of the winding.

It will be noted (see FIG. 5) that the partition 8 has a recess 80situated above the tubular winding, in the vicinity of the tubessituated at the exit from the part 2 a constituting the main exchanger.

A temperature probe 9 is mounted in this recess.

This probe is a thermal circuit breaker which is mounted sealably withrespect to the casing. For this purpose, the probe 9 is advantageouslykept in place by means of a circlip in a stainless steel cup fitted intothe recess 80, which is open to the bottom, a suitable seal providingsealing between the cup and the wall of the recess 80.

This probe is connected to the burner control and is designed to causethe burner to shut down when the temperature detected exceeds apredetermined threshold, for example 160° C.

Abnormal overheating may occur accidentally, for example in the event ofwater being absent from the tubes or in the event of poor watercirculation in the tubes, for example caused by a blockage of one ofthem.

In the absence of any safety measures, there might occur a very largerise in the temperature of the flue gases leaving the tubes placedaround the burner and coming into contact with the inside of the plasticcasing. What would happen is that the flue gases would no longertransmit their heat sufficiently to the tubes.

There might then be a problem in terms of the mechanical stability ofthe plastic and serious damage to the casing, with the latter evencatching fire.

In the variant illustrated in FIG. 8, the probe, referenced 9′, containsa heat-sensitive fusible element 92′.

The electric power circuit supplying the boiler is connected up to twoterminals 90′ and 91′ which are connected via this heat-fusible element92′.

In the event of an abnormal rise in the temperature, for example beyond160° C., this element 92′ melts and breaks the electrical circuitbetween the two terminals 91′, 90′, causing the burner control to beshut down.

FIG. 9 illustrates the circulation of the hot gases generated by theburner 6, the latter being supplied with combustible mixture G+A.

After it has been ignited, the burner generates burning gases, forexample at a temperature of 1000° C., which propagates radially outwardas symbolized by the arrows F₁.

These burning gases pass radially through the gaps in the first part ofthe exchanger 2 a from the inside toward the outside (arrows F₂).

During this passage, a large portion of the heat of the burning gases istransmitted via the wall of the tubes to the water circulating therein,with the result that the temperature of the hot gases leaving the bundlepart 2 b is, by way of illustration, around 110 to 140° C.

It will be noted that the presence of the deflector 6 prevents theburning gases F₁ from escaping axially.

The partially cooled gases then pass through the second part 2 b of theexchanger, this time from the outside toward the inside, as symbolizedby the arrows F₃.

An additional portion of the heat is thus transmitted to the watercirculating in the tubes. The temperature of the gases escaping from theappliance (arrows F₄ and F₅) is, by way of illustration, around 65 to70° C.

With regard to the water, it is generally heated up from the ambienttemperature to a temperature of around 80° C.

Of course, the water flows in the opposite direction to the flow of theflue gases, preheating taking place in the region 2 b of the exchangerand the actual heating in the region 2 a.

In the embodiment which is represented in FIGS. 10 to 12, the exchangeris not provided with a burner.

The casing contains an intake sleeve E for the hot gases, these comingfrom an external source.

This sleeve emerges on the inside of the winding of tubes 2.

This involves an arrangement which is analogous to that forming thesubject of FIG. 19 of the aforementioned European patent.

The same reference numbers have been used to denote elements which areidentical to those of the first embodiment, indexed with a prime asappropriate when the elements are similar but not identical.

A single exchanger (without preheating) is involved in this case.

The hot gases which enter the interior enclosure of the casing, via thesleeve E, escape radially from the inside toward the outside of thetubular bundle 2, heating up the fluid which circulates therein; thecooled gases escape through the sleeve 12.

The tubular elements constituting the winding may be arranged inparallel, the inlet and outlet manifolds 15′ and 16′ respectivelyproviding for their collection and distribution either at the entranceto or at the exit from the tubes.

The casing 1′ is made of plastic.

The means for mechanically retaining the bundle are similar to those ofthe first embodiment.

They comprise a set of four ties which are fastened at their ends, forexample by welding, to two plates 30, 3′.

The plate 30 situated on the intake sleeve E side is a disk whose centerhas an opening 300 in register with the gas inlet passage delimited bythe sleeve E.

The bottom plate 3′ is a disk which has not been provided with a cutout.

This disk closes off the rear part of the winding, forcing all of thehot gases to leave through the gaps between the turns.

To prevent the casing bottom wall situated facing the plate 3′, which isexposed to the hot gases, a clearance j is provided between these twoelements.

Of course, this appliance may also be equipped with a temperature probedesigned to stop hot gases being admitted when the probe detects apredetermined excessive temperature.

Returning to the first two embodiments, it should be noted that theburner employed does not necessarily have to have a cylindrical shape;it could have a flat or hemispherical shape while still remaining fixedto the door.

The weight saving obtained by using a plastic casing is around 20% withrespect to a similar appliance having the same performance but whosecasing is made of metal.

The exchanger variant illustrated in FIGS. 13 and 14 is similar, in itsstructure, to that already described with reference to FIGS. 5 to 7,which is why this structure will not be described again here.

However, as will be explained, it contains a shroud which performs thefunction of a heat shield.

Specifically, the annular part of the wall of the casing 1 whichsurrounds the winding 2 is equipped internally with a shroud 100. Thelatter is made of thin stainless steel sheet, the thickness of which isaround 0.3 to 0.4 mm, for example.

This shroud bears against the internal face of the casing, with acertain spacing j (see FIG. 13), of around 2 mm for example. Thisseparation is provided by means of a plurality of bearing studs 101consisting of cups of small size stamped into the sheet so as to formbosses projecting to the outside of the shroud. As shown by FIGS. 15 and16, which represent a developed view of the sheet in two partsconstituting the shroud, these bosses 101 have a uniform geometricdistribution in the surface of the sheet, being arranged in thisinstance as equal equilateral triangles.

The spacing j and the presence of the bosses 101, which bear against thecasing 1 by way of regions of very small area, which are virtually pointregions, makes it possible to considerably reduce the transmission ofthe heat absorbed by the shroud 100 to the wall surrounding it.

At its ends, this shroud bears, on the front side, against the facing 3,and, on the other side, against the partitions 8-8′.

Its axial length, which corresponds substantially to that of the winding2, is referenced K in FIG. 13.

In the embodiment illustrated, the shroud 100 is formed by two initiallyflat, separate parts, which are represented in FIGS. 15 and 16 andreferenced 100 a and 100 b respectively.

These are strips of stainless steel sheet of width K and of length L₁and L₂ respectively.

On its longitudinal edges, each of the strips 100 a, 100 b has a seriesof four notches 102 having a substantially semicircular or semiovalshape which is complementary with the shape of the cross section of theend portions of the tubes at the level of the wall 1 which they passthrough.

The length L₁ of the strip 100 a is significantly greater than that L₂of the strip 100 b.

The sum L₁+L₂ corresponds approximately (allowing for the spacing j) tothe circumference of the internal wall of the casing 1 against which thestrips 100 a and 100 b are pressed after they have been rounded to adaptto the curvature of the wall of the casing 1. As can be seen from FIG.14, this casing has a cross section whose contour is halfway between acircle and a square with rounded corners.

The short element 100 b is placed on that side where the mouths 20′, 21′of the tubes are situated, outside these mouths (to the left in FIG.14), while the long element 100 a is placed on the other side.

They are brought together by way of their longitudinal edges (parallelto X-X′) and tightly enclose the end portions, or mouths, of the tubesconstituting the winding 2 with a slight clearance by way of theirnotches 102, which are suitably configured and positioned for thispurpose.

As a result of their elasticity, the two strips of sheet are pressedclosely, by way of their bosses 101, against the internal face of thecasing without having to make use of specific fastening means. They thusform a shroud which, in a relatively sealed manner, insulates saidinternal face of the casing from the hot gases circulating in theexchanger, performing the function of a heat shield or isothermalshield.

If, as in the embodiment illustrated in FIG. 13, the wall of the casing1 has an inwardly pointing recess 80, which houses a temperature probe9, it goes without saying that the shroud is traversed in this region bya suitable opening into which the recessed wall portion is inserted. Inthis region, the wall of the casing, which is not protected thermally,is therefore exposed to a temperature which is higher than that of theremainder of the wall, which is protected by the shroud.

In practice, this does not present any difficulties since this regionhas a very limited area, and the excess heat arising there is evacuatedby thermal transfer toward the neighboring wall zone, which is less hot.

The presence of the shroud has the effect of lowering the temperature towhich the wall of the casing is exposed by a value of around 15 to 20°C., which makes it possible to use a less noble, and consequently lessexpensive, plastic than that which can be used with the previouslydescribed embodiments (not provided with a shroud), and/or to improvethe stability over time and also the durability thereof.

1. A condensation heat exchanger associated with a gas or fuel burner,which comprises: at least one bundle of tubes, wherein the bundleincludes one tube, or a group of tubes arranged end to end, forming ahelical winding, in which the wall of the tube or tubes is made of ahighly thermally conductive material and has a flattened oval crosssection, wherein the major axis of the cross section is perpendicular,or approximately perpendicular, to that of the helix, while the width ofa gap separating two adjacent turns is constant and appreciably smallerthan the thickness of said cross section, a fluid heated up, inparticular cold water, for circulating inside the tube or tubesconstituting said bundle; and a gas-impermeable casing, wherein thebundle is fixedly mounted inside the gas-impermeable casing, the casinghaving a sleeve for the discharge of the burnt gases, wherein theexchanger is arranged such that the hot gases generated by the burnerpass radially, or approximately radially, through said bundle via thegaps separating turns of the bundle, wherein said casing is made ofheat-resistant plastic, wherein the casing contains retaining devicesfor mechanically retaining said bundle in an axial direction of thebundle, these retaining devices being able to absorb thrust loadsresulting from the internal pressure of the fluid which circulatestherein and which tends to deform the walls thereof, while preventingthese loads from being transmitted to the casing, and wherein saidretaining devices comprise a set of ties which extend outside thebundle, parallel to the axis of the helix, and whose ends are fixed tobearing elements pressing against the two opposed faces of the bundle.2. The exchanger as claimed in claim 1, further comprising a temperatureprobe borne by said casing which is able to shut down the burner whenthe temperature prevailing inside the casing exceeds a predeterminedthreshold.
 3. The exchanger as claimed in claim 1, wherein the bearingelement situated at one of the ends of the set of ties is a thin plate,wherein the plate includes a cut out in a central part of the plate andthe plate has an annular shape.
 4. The exchanger as claimed in claim 3,wherein said plate serves as a facing which partially closes off an openface of the casing and is fastened to the latter at a periphery of thecasing.
 5. The exchanger as claimed in claim 4, wherein end portions ofthe ties pass through said facing in such a way as to project slightlyoutward, and in that the end portions are threaded such that the endportions allow a door to be removably mounted against the facing bymeans of nuts.
 6. The exchanger as claimed in claim 5, wherein said dooris fixed to the burner.
 7. The exchanger as claimed in claim 4, furthercomprising four ties arranged substantially in a square, and in that thebearing elements situated on the opposite side to said facing include apair of arcuate or bent straps configured to follow the contour of thebundle as closely as possible and pressing against two diametricallyopposed regions thereof, each strap being fastened to a pair ofneighboring ties.
 8. The exchanger as claimed in claim 1, wherein thecasing is composing of a plastic that is a composite material based onglass-fiber-reinforced or glass-flake-reinforced resin.
 9. The exchangeras claimed in claim 8, wherein said resin is a compound of polyphenyleneoxide, polystyrene and polypropylene.
 10. The exchanger as claimed inclaim 1, wherein the bundle includes two bundles of coaxial tubessituated end to end and connected to one another, one of which serves asa primary exchanger and the other as a secondary exchanger, and furthercomprising a deflecting member being sandwiched between the two bundlesand thus arranged such that the hot gases generated by the burner passfirst through the primary exchanger, passing through gaps separatingturns of the bundle from the inside to the outside of the bundle, andthen through the secondary exchanger, passing through gaps separatingturns of the bundle from the outside to the inside, after which the hotgas is discharged via said sleeve.
 11. The exchanger as claimed in claim10, wherein said deflector is fixed to said bundles of tubes.
 12. Theexchanger as claimed in claim 10, wherein the burner is mounted insidesaid bundle serving as the primary exchanger, wherein said deflector hasa discoid shape and is fixed to the end of the burner, and the deflectoris equipped at a periphery of the deflector with a thermally insulatingseal that presses against the inside of the bundle.
 13. The exchanger asclaimed in claim 1, wherein said casing is composed of two moldedhalf-shells brought together and secured to one another.
 14. Theexchanger as claimed in claim 1, further comprising a shroud arrangedoutside said bundle and inside said plastic casing, wherein the shroudacts as a heat shield, and wherein the shroud is capable of insulatingthe casing from heat emitted by the burnt gases.
 15. The exchanger asclaimed in claim 14, wherein said shroud is made from thin stainlesssteel sheet.
 16. The exchanger as claimed in claim 14, wherein saidshroud is applied to the internal surface of said plastic casing so thatthe shroud is spaced a certain distance from the plastic casing.
 17. Theexchanger as claimed in claim 16, wherein said shroud is spaced acertain distance from the plastic casing by a series of bosses stampedinto a wall of the shroud.
 18. The exchanger as claimed in claim 14,wherein said shroud includes two complementary rounded parts broughttogether so as to form an annular casing fitting against the internalsurface of said plastic casing.
 19. The exchanger as claimed in claim18, wherein mutually facing edges of the rounded parts have a row ofapproximately semicircular or semioval notches which are able to tightlyenclose the rectilinear end portions of the tube or tubes constitutingthe winding when these rounded parts are brought together.